This invention relates to an apparatus for recording color images, such as color copying apparatus, which is suitable for use in obtaining a color image of an original in color by writing optical images of three primary color components of the original in color each to the surface of one of three photosensitive drums, to form latent images on the photosensitive drums which are developed with toners of three primary colors by subtractive color mixing to produce three developed images which are then printed by transfer-printing on a sheet of support material by superposing the three developed images one over another, to provide the desired color image.
Apparatus for recording color images include a color copying apparatus for producing color copies from an original in color, a color printer for obtaining a color image by inputting digital signals for images of three colors obtained by color separation from an original in color, and a printout device of a color facsimile system, etc. In one type of these systems for recording color images known in the art, three photosensitive drums are each exposed to one of three optical images of an original in color produced by color separation by means of an optical system relying on slit exposure or exposed to one of three optical images of an original in color obtained by color separation by inputting a digital signal for each one of the three optical images to drive a respective optical writing head, to produce three electrostatic latent images on the three photosensitive drums which are developed with toners of three primary colors by a subtractive color mixing method to produce three toner images of different colors which are printed by transfer-printing on a sheet of support material by superposing them one over another to provide a color image of the original.
Generally, one of the most important requirements that should be met in color copying apparatus of the superposed image transfer-printing system is that deviation in position of color regions of the three developed images of different colors from each other should be avoided. To this end, an array of gradient-index rod lenses (GIRL) which enables an image of the same size as the original to be produced is used as an image forming optical system for projecting optical images on the photosensitive drums by moving the original, to ensure that the color image obtained is of the same size as the original. However, an image formed by an array of GIRL is a reversed image of the original in which the right side and the left side are reversed as is the case with a mirror image while an image formed by a single image forming lens is a reversed image of the original in which not only the right side and the left side but also the top side and the bottom side are reversed. Therefore, when an optical image is projected directly on each photosensitive member from the array of GIRL, it is necessary to move the peripheral surface of the photosensitive drum in the same direction as the original in the position in which the peripheral surface is exposed to the optical image, to form a continuous image of the original on the photosensitive drum. When an attempt is made to directly print the developed image from the photosensitive drum on a sheet of support material in a transfer-printing position which is diametrically opposed to the position in which the peripheral surface is exposed with respect to the center axis of the photosensitive drum, the direction in which the sheet of support material is conveyed is opposite the direction in which the original is moved. However, when the three photosensitive members have their peripheral surfaces exposed to optical images of an original with a suitable time lag in positions which correspond to each other while the original is moved above the photosensitive drums, it is impossible to superpose three toner images of different colors one over another unless they are transferred from the photosensitive drums to a sheet of support material with a suitable time lag while moving the sheet of support material in the same direction as the original.
It has hitherto been usual practice to insert a mirror between the end of each array of GIRL through which a light ray emerges and the position on each photosensitive drum in which the photosensitive drum is exposed to the optical image so as to reverse the direction of movement of the optical image, the mirror being constructed in the form of a multilayer mirror to serve concurrently as a color separator. However, when this type of mirror is used, the image formed on each photosensitive drum is a normal image, so that when the image is developed and directly printed on a sheet of support material, a color image of the original produced on the sheet of support material is reversed from the image of the original. Thus, it has hitherto been necessary to use an intermediate transfer-printing member to which the image is transferred temporarily from the photosensitive drum before all the images are printed by transfer-printing on a sheet of support material by superposing one over another to produce the desired color image of the original. This has made the apparatus complex in construction and large in size, causing a rise in cost.
In the case of a color printer or a color facsimile system in which optical writing heads are driven when digital signals are inputted thereto to write optical images to three photosensitive drums, signals for the three primary colors are usually taken out simultaneously for forming toner images of three different colors each on one of the three photosensitive drums, and the toner images of different colors thus formed are printed by transfer-printing on a sheet of support material which is fed in a manner to be brought successively into contact with the positions on the three photosensitive drums in which the three toner images of different colors are formed. To perform the operation described hereinabove, it is necessary that the optical images be written on the three photosensitive drums with a suitable time, lag and a memory for storing the signals for the different colors temporarily must be provided. Such memory is very expensive, causing a rise in the cost of the apparatus.